1. Field of Invention
The present invention relates to electro-optical backlighting panels for use in illuminating flat panel displays, optical transparencies, film structures and the like.
2. Brief Description of the Prior Art
Most portable computer systems include a flat liquid crystal display(LCD) panel for directly viewing video imagery displayed thereon. Portable computer systems of this type include notebook, laptop, and palmtop computers.
In general, prior art LCD display panels have essentially the same basic construction, in that each includes a conventional backlighting structure affixed to the rear surface of either a passive or active matrix LCD. panel. Several different backlighting panel designs are described in the technical paper “New Backlighting Technologies for LCDs” by Kevin J. Hathaway, et al., published at pages 751-754 in SID 91 Digest. In recent times the “light pipe” backlight design. in particular. has been used in many commercially available notebook computers.
Specifically, prior art “light pipe” backlight assemblies are constructed from a rectangularly shaped light guiding panel, typically fabricated from an acrylic plastic sheet having a thickness of about 4 millimeters or so. Along the opposite side edges of the acrylic sheet, a pair of miniature fluorescent light tubes are mounted within suitably designed light reflective mounts. The function of the fluorescent light tubes is to produce and direct incoherent light into the interior of the light guiding panel within which the light is typically bounded by the well known principle of “total internal reflection”. Under ideal conditions, light will not leak out of the surfaces of the acrylic plastic sheet. However, light can be extracted or leaked out from these surfaces by forming therein scratches, undulations, or any other means of locally altering the critical angle for total internal reflection. By achieving light extraction in this manner, the backlighting panel can be used to illuminate a LCD panel.
In order to compensate for the decrease in light intensity in the light guiding panel at distances away from the fluorescent tubes, a light extracting pattern is permanently formed on one or both surfaces of the light guiding panel. Typically, the light extracting pattern is realized as a dot pattern permanently embossed or sandblasted upon the front surface of the acrylic light guiding panel. In order to achieve light intensity compensation along the light guiding panel, the density of the dot pattern is made to increase quadratically with distance from the fluorescent light tubes. With this construction a constant backlighting brightness is maintained across the light guiding panel.
In order to integrate (i.e. diffuse) the spotted distribution of light emanating from the light extracting pattern towards the LCD panel, a first light diffusing sheet is placed on top of the light guiding panel. Typically, the first light diffuser is made from one or more thin sheets of translucent plastic, attached to the front surface of the light guiding panel. In most commercial “light pipe” backlight designs, a second light diffusing sheet is placed over the rear surface of the light guiding panel to diffuse the spotted distribution of light emanating from the permanently formed light extracting pattern towards the reflective surface disposed behind the light guiding panel. Typically, the second light diffusing is made from one or more thin sheets of translucent plastic attached to the rear surface of the light guiding panel. Together, the light guiding panel, fluorescent light tubes, light diffusing sheets and the reflective layer cooperate to produce a plane of backlight having a uniform spatial intensity for optical processing by the LCD panel affixed to the backlighting panel.
While the prior art backlighting panel design described above has proven useful in the direct viewing of visual imagery on LCD display screens, its permanently formed light extracting pattern renders it unsuitable in projection viewing modes of operation. This fact is best illustrated by example.
In the recently introduced notebook computer, marketed under the tradename “Cruiser” by EMCO/REVERED Technologies, Inc., the above-described “light pipe” backlighting panel design is used to construct a portable computer system having both direct and projection viewing modes of operation. In the direct viewing mode, the prior art backlighting panel is positioned against the active-matrix LCD panel. Each time the user desires to operate the notebook computer in its projection viewing mode, the user must mechanically reconfigure the notebook computer by physically removing the prior art backlighting panel in order to reveal the active matrix LCD panel, and provide an optically clear path for the light rays to pass therethrough.
While the above-described notebook computer provides both direct and projection viewing modes, it suffers from a number of serious shortcomings and drawbacks which make it less than commercially attractive.
In particular, the light guiding panel in the prior art backlighting panel has permanently formed light diffusing surfaces, and thus must be physically removed to permit externally projected light to pass onto the active-matrix LCD panel without diffusion during its projection viewing mode of operation. The need to physically remove the entire backlighting panel from the Cruiser computer during its projection viewing mode, has made it virtually essential to fit the backlighting panel loosely against the LCD panel during the direct viewing mode. This design requirement necessarily compromises the “optical coupling” between the backlighting panel and the LCD display panel when the system is operated in its direct viewing mode. Consequently, during the direct viewing mode of operation, images are dimly displayed on the LCD display panel of the Cruiser computer.
From a practical standpoint the poor optical coupling efficiency in the display panel assembly of the Cruiser computer necessitates higher luminous output from its fluorescent tubes in order to illuminate the display panel to a particular intensity level. Regrettably. this requirement results in increased power consumption which is highly undesirable in practically all portable computing applications.
Thus. there is a great need in the art for an improved backlighting panel construction that can be used in the illumination of optical transparencies, film structures, and the flat display panels of computer-based systems having direct and projection viewing modes.